1. Field of the Invention
The present invention relates to a printing method and printing apparatus in which a print head is scanned in a direction transverse to the feeding direction of a printing medium to achieve printing of an image line by line.
2. Description of the Related Art
Heretofore, printing apparatuses using various image formation means have been practically used, and among these, an ink-jet type is relatively inexpensive, compact in structure, low in noise, and being used in a variety of applications from personal to office use.
As a method of image formation in a printing apparatus, a serial scan system has been generally known. In the serial scan system, while the print head is being reciprocally scanned, the printing medium is fed in a direction perpendicular to the scanning direction of the print head, intermittently in an amount equal to the printing width of one scan, to sequentially form the image line by line.
The serial scan system is also widely applied in the area of ink-jet type recording apparatus.
Heretofore, the ink-jet type printing apparatus has been disadvantageous in that the image formation speed is slower as compared with an electrophotographic type. However, recently, with improved integration ink ejection ports of the print head, some of print heads having 512 ink ejection ports arranged in a recording density of 400 dpi (dot per inch) have been practically applied, which achieves an image formation speed comparable to the electrophotographic system.
However, when an image is formed by serial scanning, it is necessary that the printing medium is fed exactly in a predetermined amount in order to obtain an image of good quality. To realize such feeding, a feeding unit of the printing medium is normally composed of a roller pair disposed at the upstream side or downstream side with respect to the feeding direction in the vicinity of the print portion of the printing medium, a drive source to drive the roller pair, drive transmission means for transmitting the driving force from the drive source to the roller pair, and the like. As the drive source, a pulse motor which can easily control the feeding amount of the printing medium is generally used, and as the drive transmission means, a pulley and rubber belt or the like is generally used.
However, if the feeding amount of the printing medium becomes in is shortened even slightly, image overlapping (black stripes) will occur, or if the feeding amount is excessive, image discontinuity (white stripes) will occur. Such error in the feeding amount becomes visually notable when occurring even in an amount of about a half picture element (about 30 .mu.m) for a 400 DPI image, which considerably degrades the image quality. Causes of the error include machining precision of the feeding roller and driver pulley for feeding the printing medium, stopping precision of the drive source, and machining precision of the teeth of belt or pulley; in particular, error due to an eccentricity of the feeding roller and drive pulley on the feeding roller shaft is important.
To reduce the effect of the eccentricity, it is best that the circumferential length of the feeding roller is made equal to the printing width of one scan. That is, for a print head of 400 dpi, 512 ink ejection ports, since the printing width of one scan is 32.512 ink ejection ports, effect of eccentricity between the feeding roller and the drive pulley which is linked to the shaft thereof can be almost eliminated when a roller is used as the feeding roller which satisfies the requirement that its diameter of 10.349 mm corresponds to the circumferential length. In this case, when a reduction ratio between the feeding roller and the drive pulley on the same shaft is set to an integer, vibration of the drive pulley due to the motor and feeding errors generated from the stopping precision of the motor itself can be almost eliminated, and a practically sufficient feeding precision can be realized where the printing medium is fed in an amount equal to the printing width of one scan of the print head.
However, if the printing medium feeding amount is set always to a constant value, depending on the size of the printing medium, a large blank may occur at the rear end of the image, the blank amount may differ with different sizes of the printing medium. Then, in a printing apparatus which can handle different sizes of the printing medium, one of a plurality of printing movments to one printing medium is printed in a smaller printing width than usual to adjust the length of the blank.
Feeding during the one printing movement is referred to as a fractional feeding, the length of which can be calculated as a remainder of the integer portion of {(L-a-b)/X}, herein X is a printing width by a normal one scan of the print head, L is a total length of the printing medium in the feeding direction, a is a front blank, and b is a rear end blank. For example, when printing is made on a printing medium of A4 size which is fed along a longitudinal direction thereof, that is, 297 mm in total length, by an apparatus with a print head of 400 dpi, 512 ink ejection ports, that is, a normal printing width X by one scan of 32.512 mm, a front blank a of 2 mm, and a rear end blank b of 10 mm, the number of printing movements is 9, of which one movement may be made in a fractional printing (hereinafter referred to as fractional recording) with a printing width of 24.904 mm which is smaller than usual.
With the above fractional recording, image blank on the printing medium can always be made to a constant size irrespective of the size of the printing medium, but if the fractional recording is made at the top print line (hereinafter referred to as top line) at the downstream side with respect to the feeding direction or an intermediate print line (hereinafter referred to as intermediate line), it becomes necessary to feed the printing medium by a distance equal to the width of the fractional recording. Feeding of the printing medium according to the position where the fractional recording is made will be described with reference to the accompanying drawings.
(A) When fractional recording is made in the top line using the ink ejection ports at the downstream side of the print head
In FIG. 16A, a printing medium P fed from the right side in the direction of arrow C stops at the position where the leading end thereof advances by 2 mm blank from the print area (hatched area in FIG. 16A). A print head 51 makes fractional recording using 392 ink ejection ports at the downstream side with respect to the feeding direction of the printing medium P corresponding to the 24.904 mm fraction. After completion of the fractional recording, the printing medium P is fed in the direction of arrow C in FIG. 16B by the fraction equal to the present printing width. Thereafter, printing using all ink ejection ports of the print head 51 and feeding of the printing medium P by the distance of the entire ink ejection ports width are repeated to achieve printing. The numeral 52 indicates a feeding roller.
(B) When fractional recording is made in the intermediate line using the ink ejection ports at the downstream side of the print head
Until fractional recording is made, printing of the entire ink ejection ports width and feeding of the printing medium P of the entire ink ejection ports width are repeated. During fractional recording, first, after making fractional recording using the ink ejection ports for the fraction at the downstream side of the print head, the printing medium P is fed in an amount equal to the fractional recording amount. Thereafter, printing using all the ink ejection ports and feeding of the printing medium P of the entire ink ejection ports width are repeated.
(C) When fractional recording is made in an intermediate line using the ink ejection ports at the upstream side of the print head
As shown in FIG. 17A, after completion of printing of a line immediately before the fractional recording is made, as shown in FIG. 17B, the printing medium P is fed in an amount equal to the width of fractional recording to be made next. Then, after making fractional recording using the ink ejection ports for the fraction at the upstream side of the head, the printing medium P is fed in an amount equal to the entire ink ejection ports width. Thereafter, printing using all the ink ejection ports and feeding of the printing medium P of the entire ink ejection ports width are repeated.
(D) When fractiontial recording is made in the last line using the ink ejection ports at the upstream side of the print head
After printing using all the ink ejection ports and feeding of the printing medium P of the entire ink ejection ports width are repeated, for only before fractional recording of the last line, the printing medium P is fed in an amount of the fractional recording width. Then, the fractional recording is made using the ink ejection ports for the fraction at the upstream side of the print head 51. Since this is the last printing line, thereafter the printing medium P is fed to the downstream side to complete printing.
In all of the above described examples (A) to (D), the printing medium P is fed once in an amount of the fractional recording width. However, in feeding, eccentricity of the drive pulley and the feeding roller 52 is not offset. For example, as shown in FIG. 18, when the fractional recording width is about 16 mm which is a half circumferential length of the feeding roller 52, if the feeding roller 52 with a diameter of about 10 mm has an eccentricity of only 10 .mu.m, in the worst case, a feeding error of .+-.20 .mu.m will occur. Since the eccentricity occurs also in the drive pulley, precision in feeding of the fractional recording width is considerably deteriorated as compared with normal feeding.
Methods which do not make feeding of the fractional recording width include the following.
(E) When fractional recording is made in the last line using the ink ejection ports at the downstream side of the print head
In the feeding methods in the above-described examples (A) to (D), as Shown in FIG. 19, where M is a distance between the feeding roller 52 and the upstream end of the printing area, the distance M is a minimum rear end blank that occurs in the printing medium P. In the present example, to maintain the rear end blank at the same length M, as in FIG. 20, fractional recording must be made after the rear end of the printing medium P is passed through the feeding roller 52. For this purpose, in order to make fractional recording of the last line while the printing medium P is pinched by the feeding roller 52, the blank at the rear end of the printing medium must be increased by one line of the printing width, and, as a result, there occurs a problem in that the effective image printing area in the printing medium P is substantially reduced.
As shown in FIG. 21, when a feeding roller 53 is disposed also at the downstream side of the printing unit, rear end blank of the printing medium P can be reduced. However, even with this arrangement, during feeding of the printing medium P before printing the last line, the rear end of the printing medium P comes out of the feeding roller 52 at the upstream side. Therefore, a feeding precision of the printing medium P in the specific case depends on the feeding roller 53 at the downstream side of the printing unit In the case of an ink-jet printing system, the printing medium P at the downstream side undergoes elongation due to the ink with time as compared with the printing area in it. The elongation amount greatly depends on the sheeting direction of the printing medium P, the amount of ejected ink (image density), and the type of printing medium P, for example, an elongation of 0.1% (30 .mu.m) of the printing width occurs when the printing medium P is a coated paper with small elongation, and an elongation of 1% (300 .mu.m) of the printing width occurs in ordinary paper.
With such an elongated printing medium P, even when the feeding roller 53 at the downstream side of the printing unit is rotated by a predetermined amount, the actual moving amount of the printing medium P is reduced by the elongation amount. That is, when fractional recording is made with the rear end of the printing medium P not pinched by the upstream side feeding roller 52, a feeding error of a maxim of 300 .mu.m will occur. Furthermore, since the error itself varies with the type of printing medium P and the printing density, it cannot be corrected.
The above description can be summarized as follows.
(A) Fractional recording in the top line using the ink ejection ports at the downstream side of the head: deterioration of precision in feeding of the printing medium for fractional recording.
(B) Fractional recording in an intermediate line using the ink ejection ports at the upstream side of the head: deterioration of precision in feeding of the printing medium for fractional recording.
(C) Fractional recording in an intermediate line using the ink ejection ports at the downstream side of the head: deterioration of precision in feeding of the printing medium for fractional recording.
(D) Fractional recording in the last line using the ink ejection ports at the upstream side of the head: deterioration of precision in feeding of the printing medium for fractional recording.
(E) Fractional recording in the last line using the ink ejection ports at the downstream side of the head: increase of the rear end blank or deterioration of its dimensional precision which leads to deterioration of precision in feeding of the printing medium for fractional recording.
The above-described examples are those of simplest image formation, and in an apparatus which reads an object image and makes a copy, the above fractional recording may occur frequently.
An example thereof is an image repeat function in which, as shown in FIG. 22, a part of the object image is extracted and repeatedly printed. The image area of image repeat is generally designated by the operator, when an object image of a width Y and a length K is repeated in the feeding direction of arrow C of the printing medium P as shown in FIG. 23A, the length K is not always guaranteed to be an integer multiple of printing width X of the entire ink ejection ports width of the print head. When the length K is smaller than the printing width X of the entire ink ejection ports width of the print head, in the past, a process has been repeated in which after making fractional recording of one line using the ink ejection ports corresponding to the width K at the downstream side of the print head, the printing medium P is fed by a distance equal to the length K. Further, as shown in FIG. 23B, when the length K is greater than the printing width X, an integer multiple of the printing width x is subtracted from the length K, and the same fractional recording is made for the remainder print portion. Therefore, when forming such an image, fractional recording must be made many times during printing on a sheet of the printing medium P, and deterioration of feeding precision of the printing medium P during fractional recording has been a major problem.